Technological advances in directional drilling within the oil industry have enabled wells to be completed with long horizontal sections in contact with the reservoir. Such long horizontal well bores, often in excess of 1,000 m, permit fluids to be injected into or produced from a much greater portion of the reservoir, than would be possible from a vertical well, with commensurately greater recovery of petroleum from a single well. The greater petroleum recovery possible from such wells, more than justifies the increased cost of drilling and completing the horizontal well section. Additionally, horizontal wells require fewer wellheads with less surface disturbance to exploit the same reserves, providing a collateral environmental benefit. These reasons are strong motivators to ensure technically and economically viable products are available to complete these wells.
For such reservoirs the horizontal section is often completed with slotted steel tubulars (referred to as slotted liners) to prevent closure of the hole through collapse and to function as a screen or filter permitting flow of injected or produced fluids across the tubular wall while excluding solids. The present invention was conceived as a means to improve both the technical and commercial viability of slotted liners, particularly needed where the reservoir material is comprised of weak fine-grained materials.
To function effectively as a filter and structural support member in fine-grained reservoirs, and to be sufficiently rugged to endure installation handling loads, the slotted liner design is driven by three somewhat competing needs. To ensure adequate solid particle exclusion, the slot width must be on the order of the smaller sand grain size. This is generally true even where fluids are injected, because the effective radial stress in the sand tends to force sand grains into the well bore, even though the fluid flows out. For reservoirs comprised of very fine-grained material, slots less than 0.15 mm may be required. But small slot widths tends to increase flow loss, therefore a greater number of slots are needed per unit of contacted reservoir area to maintain flow capacity, while the greater number of slots must be accommodated without undue loss of structural capacity. The industry also recognises advantages for production applications, if the slot has a ‘keystone’ shape, i.e., the flow channel through the tubular wall diverges from the external entry to internal exit point. This geometry reduces the tendency for sand grains to lodge or bridge in the slot, causing it to plug and restrict flow.
As pointed out by Hruschak in U.S. Pat. No. 6,112,570, the methods usually used to cut slots through the wall of steel tubulars having a wall thickness great enough to provide adequate structural support in horizontal wells, are not readily applicable for widths less than 0.4 mm. Hruschak then goes on to disclose a method where this limitation is overcome by deforming or forming one or both of the external edges of a longitudinal slot, placed in the wall of a steel tubular, to narrow the slot width along its exterior opening. This method relies on applying pressure along at least one of the longitudinal edges, preferably by means of a roller, where such pressure is sufficient to cause local plastic deformation of the metal, and thus permanently narrow the slot to a desired width. As recognized by Hruschak and others using similar methods, such as Steps in U.S. Pat. No. 1,207,808, this method of forming the exterior longitudinal edges of a slot, has the added advantage of producing a ‘keystone’ slot shape where the through-wall channel shape diverges from the exterior to interior edges of the slot. Processes employing such methods to narrow the slot width by applying pressure at or along a slot edge to plastically deform it inward are referred to as seaming.
It will be apparent to one skilled in the art that methods of reducing the slot width by the application of pressure along or parallel to the edge of a slot, as described by Steps or Hruschak, will be sensitive to the location where pressure is applied. Specifically, the amount by which the slot width is decreased depends strongly on the distance between two parallel lines, one coinciding with the slot centre and the second with the longitudinal force centre of the pressure applied along the slot length. The alignment tolerance may thus be defined as the allowable range of distance between these two lines to meet the required tolerance in final slot width. The required tolerance in slot width is typically in the order of +/−0.02 mm. With practical seaming tooling, the associated alignment requirements can be in the order of +/−0.1 mm.
Hence such methods require relatively accurate alignment of the load application means, such as a forming roller, with respect to the circumferential position of longitudinal slots. To implement this method in a mechanised process capable of forming a large number of slots on full-length tubulars, therefore requires considerable sophistication to co-ordinate the positioning of tooling required to perform the respective cutting and seaming operations if conducted sequentially in a single machine. Even further sophistication is required if the cutting operation is performed independent of the seaming. The capital cost associated with such machinery make it difficult to obtain economically viable rates of production on full-length tubulars, particularly so if the slotting is conducted independent of the seaming.
However it is particularly attractive to decouple the cutting and seaming operations as this allows seaming to be conducted on tubulars slotted by various independent suppliers, improving the economics of supply. In this case, circumferential positioning of the longitudinal seaming tools must account for a degree of randomness in the circumferential distribution of slots obtained from typical suppliers of slotted liner that significantly exceeds the allowable alignment tolerance.